HUNCHBACK-INDEPENDENT SILENCING OF LATE UBX ENHANCERS BY A POLYCOMB GROUP RESPONSE ELEMENT

Drosophila homeotic genes are kept silent outside of their appropriate expression domains by a repressive chromatin complex formed by the Po lycomb Group proteins, In the case of the Ubx gene, it has been propos ed that the early repressor HE, binding at enhancers, recruits the Pol ycomb complex and specifies the domain of repression. We show that som e Ubx enhancers are activated after blastoderm. If a Polycomb Response Element (PRE) is combined with such late enhancers, repression of a r eporter gene can be established everywhere in the embryo, irrespective of the presence or absence of hunchback protein, If, however, these l ate enhancers are combined with a Ubx early enhancer, as well as a PRE , repression is established only where the reporter gene was inactive at early stages. These results imply that the Polycomb complex is not dependent on hunchback and suggest that the pattern of silencing refle cts rather the state of activity of the gene at the time the Polycomb complex is formed.